Instead of outsourcing custom designed parts, most people buy components that are already designed and available such as bearings and blocks from cncrouterparts.com. Or if you want higher grade stuff, you can buy bearing blocks, ball screws and linear bearings on ebay from Chinese vendors such as linearmotionbearings2008 and others. This lowers your cost as the setup costs are spread over a much larger number of people.
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html